Welding has been an indispensable process in the manufacture of large steel structures such as smokestacks, bridges and pressure vessels. Because such large structures cannot easily be inverted during production, some portions must be welded in the horizontal position.
In butt welding, four welding positions may be used: the flat position, the horizontal position, the vertical position and the overhead position. In fillet welding, four welding positions also may be used: the flat position, the horizontal position, the vertical position and the overhead position.
Generally, "welding in the overhead position" refers to welding a joint from below with the axis of the weld approximately horizontal. "Welding in the flat position" refers to welding a joint from above with the axis of the weld approximately horizontal. "Welding in the horizontal position" refers to welding a joint from the side with the axis of the weld approximately horizontal. "Welding in the vertical position" refers to welding a joint with a vertical axis of the weld lengthwise along the vertical.
In the disclosures which follow, the phrase "welding in the horizontal position" will refer to welds such that the angle of the parent material with respect to a horizontal surface is either approximately vertical or within the range of 45.degree. to 90.degree.; and the weld line is either approximately horizontal or its angle with respect to a horizontal surface is within the range of 0.degree. to 45.degree..
In other words, the present invention is also applicable in a range of welding positions which might be called intermediate between the common overhead, flat and horizontal positions. In this discussion we shall use the phrase "horizontal position" in the wider meaning specified above.
When a joint is welded in the horizontal position, as shown in the sketch in FIG. 9, the weight of the molten metal gives weld bead 7A a tendency to sag, with the result that the upper portion of the open end of the bead is undercut (A in FIG. 9) while the lower portion has an overlap (B in FIG. 9). This may lead to welding defects such as inadequate fusion when the subsequent layer is welded. This is why such welding is usually executed by a highly-skilled welder using a semi-automated process.
Even if this welding process is automated, the welding apparatus must have the same level of capability as a skilled welder mentioned above. A number of weaving mechanisms will be necessary. And since such mechanisms are still not enough, the process must be conducted at a low temperature with a resulting low efficiency. This stands in the way of making the welding process more efficient.
With the techniques used in the prior art, then, welding defects such as an undercut of the upper portion of the open end of the bead or an overlap of the lower portion were likely to occur when a joint was welded in the horizontal position.